LINE-SHAPES FOR LASER-INDUCED COLLISIONS

The two-state model of laser-induced collisions introduced by Gudzenko and Yakovlenko is shown to lead to cross sections with a universal behavior in terms of the variables z=|C3|E0v-35|C6|-25 and d=δ|C6|15v-65sgn(C6). In terms of a physical description the dimensionless frequency-detuning variable is d(detuning of the laser from the large-R resonance) (time of collision at the Weisskopf radius). The dimensionless variable z is independent of laser frequency and measures the power dependence of the cross section. It is in fact proportional to tC3E0dtR(t)3 evaluated at an impact parameter given by the bν=(C6v)15Weisskopf radius = impact parameter where the phase shift due to the Van der Waals potential becomes π. Above, C3E0R3 is the coupling parameter at intranuclear separation R and E0 is the laser field amplitude. The cross section is of the form σ=(|C6|v)25H(d,z), where H(d,z) is tabulated in detail. For large laser fields (i.e., z>2), the line shape for collisions at a particular relative velocity v, laser field amplitude E0, and detuning (from the large-R resonance frequency), σ becomes symmetric about δ=0 with the width decreasing with increasing laser power. The physical reason for the symmetric H(d,z) at large z is shown to be the decreased importance of curve-crossing effects for large positive d corresponding to the onset of adiabatic behavior and the increased importance of contributions to σ from such large impact parameters that the Van der Waals shifts can be neglected. Correspondingly, at large z the linewidth is due entirely to time-of-collision effects. When 2, both the long-range version of the atom-atom interaction and the assumption of straight-line orbits are excellent because of the dominant contribution to δ from impact parameters > 15. © 1979 The American Physical Society.